gdbserver, remote: introduce "id_str" in the "qXfer:threads:read" XML

GDB prints the target id of a thread in various places such as the
output of the "info threads" command in the "Target Id" column or when
switching to a thread.  A target can define what to print for a given
ptid by overriding the `pid_to_str` method.

The remote target is a gateway behind which one of many various
targets could be running.  The remote target converts a given ptid to
a string in a uniform way, without consulting the low target at the
server-side.

In this patch we introduce a new attribute in the XML that is sent in
response to the "qXfer:threads:read" RSP packet, so that a low target
at the server side, if it wishes, can specify what to print as the
target id of a thread.

Note that the existing "name" attribute or the "extra" text provided
in the XML are not sufficient for the server-side low target to
achieve the goal.  Those attributes, when present, are simply appended
to the target id by GDB.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Approved-By: Simon Marchi <simon.marchi@efficios.com>